1. Field of the Invention
This invention relates to an optical record reproducing apparatus for automatically controlling the amount of light generated by a reproducing light source, when this light source is driven.
2. Description of the Related Art Statement
Such a light emitting device as an LED or laser diode is often used for a reproducing light source in an optical record reproducing apparatus. The light emitting device is generally high in temperature dependence in each characteristic that, when it is driven at a fixed electric current, the light output easily varies under the influences of the heat generation of the device itself and the ambient temperature. Therefore, as shown in the publication, for example, of Japanese patent, application publication No.10481/1979, the light generated by a reproducing amount of light source of an optical record reproducing apparatus is automatically power controlled so that the light output can be fixed. The automatic power control (abbreviated, as APC hereinafter) is a feedback control wherein the amount of emitted light generated by a reproducing light source is monitored and is compared with a predetermined reference value, so that the light source driving current is adjusted on the basis of the difference between the emitted light amount and the predetermined reference value.
In the optical record reproducing apparatus currently in use making such APC, the following methods are known: (1) a method wherein, during the reading operation, an APC circuit loop is always closed and a feedback is always applied; and (2) a method wherein, during the reading operation, an APC circuit loop is closed at intervals of a fixed time and a feedback is intermittently controlled.
When such APC is made, if a difference is produced between the emitted light amount of the reproducing light source and a reference objective value to be a reference value, the light source driving current is controlled to follow the objective value in response to this difference and the emitted light amount is kept fixed.
However, in a case in which the APC described above is made, the emitted light amount of the reproducing light source fluctuates during the period after the APC starts the operation until it converges to the objective value. Therefore, in the optical record reproducing apparatus currently in use, the fluctuation of the emitted light amount is generated during the reading operation. That is to say, in the course of following the objective value, the emitted light amount of the reproducing light source fluctuates by application of the APC and, therefore, during the reading operation, a difference is produced between the objective value and the light source emitted light amount, or the emitted light amount fluctuates at regular intervals. It has been a problem that a defect such as a reading error is caused by the fluctuation of the emitted light amount.